1. Field of the Invention
The present invention relates to an image forming apparatus, and more specifically, relates to an image forming apparatus for developing an electrostatic latent image formed on an image carrier and transferring the developed image onto a sheet.
2. Description of Related Art
Conventionally, in the field of electrophotographic copying machine and laser printer, in order to stabilize the picture quality, AIDC (auto image density control) is made in such a manner that a test pattern is formed on a photosensitive layer under predetermined image forming conditions, that the density of toner attracted onto the test pattern is detected optically by a sensor and that the detected density is fed back into the image forming conditions (toner supply to a development tank or charging voltage of the photosensitive layer, an exposed light amount, a development bias voltage, etc.).
In the AIDC, in order to stabilize the picture quality more accurately, it is preferable that test patterns of plural densities (for example, three types: low-density pattern P1, intermediate-density pattern P2 and high-density pattern P3) are formed, and their densities are detected so that the detected densities are fed back into correction control of the image forming conditions. However, if in multi-copying (plural copies are formed successively by one copy start signal), three types of test patterns are formed in an intro-image area between image forming areas and are subjected to density detection, the intro-image area is widened, thereby causing problems that a copy speed is lowered and that toner consumption is increased.
Therefore, in a conventional example, as shown in FIG. 11a, the intermediate-density pattern P2 is formed for AIDC every time when an image is formed on one sheet, and all the patterns P1, P2 and P3 are formed for AIDC when the last copy in multi-copying has been made. However, in this method, when the number of copies made in multi-copying is increased (detection of only one kind of the pattern P2 is continued), it is inevitable to lower the accuracy of the detection.
As its reform measure, as shown in FIG. 11b, a method, in which every time multi-copying on a specified number of sheets is completed (for example, copying on 25 sheets is completed), the three types of test patterns P1, P2 and P3 are formed for AIDC, is also suggested. However, this is not a drastic solution, since AIDC based on detection of only one type of the intermediate-density pattern P2 is continued during printing on the specified number of sheets.